A group of four senior investigators has used biochemical, somatic cell genetic, and cell biological tools to investigate the structure and function of immunologically important cells. Homogeneous cultured cell lines have been developed and have been used in many of the studies carried out. The specificity of macrophage Fc receptors has been investigated using monoclonal antibodies and mutant immunoglobulins. The biochemistry of macrophage function has been studied using somatic mutants of macrophage cell lines. Mouse myeloma cells have been used to study the expression of antibody genes and to examine structure-function relationships in immunoglobulin molecules. Using naturally occurring histogenic mutants, the chemical basis for the T-cell recognition of H-2 antigens has been examined.